.... and when the waiter accidentally presses that button and changes the card which your business lunch is being charged to?
Perhaps do the card selection from the app? Or at least an option to require confirmation in the app?
You go to the app on your phone and select the card you want. The coin will now function as that card for the next 5 minutes only. Otherwise, the coin is nothing.
Now even if your wallet is stolen, without your phone (+ unlock code, etc.) they don't have any of your cards!